Brushless dc motor and rotor magnet

ABSTRACT

A brushless dc motor comprising a stator, and a rotor incorporating a hollow cylindrical permanent magnet. The rotor magnet has sectors magnetized radially in opposite directions to define poles of opposite polarity extending around the circumference of the magnet. The magnet is completely magnetized so as to leave no non-magnetized sectors between the poles of the magnet. The magnet has grooves in its surface facing the stator, the grooves extending in the longitudinal direction of the magnet. Each groove is offset from, or non-symmetrical with respect to, the center of the rotor pole in which it is located.

The present invention relates to a brushless dc motor, comprising astator having magnetic poles with coils and interpoles, a rotor magnetof cylindrical shape, radially magnetized, having magnetic grooves onits surface facing the stator, and a rotor position sensor, and moreparticularly to the rotor magnet used in a brushless dc motor.

A technology whereby a combination of a stator having interpoles as wellas magnetic poles with coils, and a rotor magnet with a specialmagnetized pattern, keeps the electromagnetic force from vanishing at acertain rotor position, stabilizes starting of a brushless dc motor. Themagnetized pattern formed on the rotor magnet divides the rotorcircumference by 3 P / 2 and is arranged in the order of N pole, S pole,and non-magnetized area. This technology is set forth, for example, inU.S. Pat. No. 3,299,335. As for technologies for sensing rotor position,one of them uses a disc with a slit directly linked with a rotor todetect the rotor position by a photoelectric converter, while anotheruses a separate magnet on a rotor to detect the rotor position by amagnetoelectricity converter, e.g., a Hall effect device.

FIG. 10 illustrates a prior art inner-rotor brushless dc motor of 4-poleconstruction with a single magnetoelectricity converter for rotorposition sensing The stator comprises a motor case 1, a stator yoke 2,four stator magnetic poles arranged at regular intervals 3a, 3b, 3c, 3d,coils wound around each of the stator magnetic poles 4a, 4b, 4c, 4d, andinter-poles 5ab, 5bc, 5cd, 5da, arranged halfway between the statormagnetic poles. The rotor consists of a torque-generating rotor mainmagnet 7 and a rotor position sensing magnet 8. A magnetoelectricityconverter 6 is fixed opposite the rotor position sensing magnet 8. Therotor main magnet 7 is magnetically divided on its circumference atangles of every 60 degrees. The sections are S pole, non-magnetizedarea, N pole, S pole, non-magnetized area, and N pole, respectively, inthis order.

The magnetic poles of the rotor main magnet 7, and those of the rotorposition magnet 8, are positioned as shown in FIG. 11.

Where the rotor position sensing magnet 8 and the magnetoelectricityconverter 6 are positioned as shown in FIG. 10, stator magnetic poles3a, 3c are excited to N poles and the other stator magnetic poles andinter-poles are excited to S poles, when coils 4a, 4c are energized.Then, the N poles of rotor main magnet 7 and the N pole magnetizedstator magnetic poles 3a, 3c repel each other, causing the rotor torotate clockwise (as indicated by an arrow). After the rotor rotatesthrough a 90-degree arc, coils 4a, 4c are deenergized and at the sametime coils 4b, 4d are energized. Then, stator magnetic poles 3b, 3d areexcited to N poles and the other stator magnetic poles and inter-polesare excited to S poles. Consequently, a magnetic repulsion, between theN poles of rotor main magnet 7 and the N pole-magnetized stator poles,keeps the rotor rotating clockwise.

Each time the rotor rotates through a 90-degree arc, the magnetizingconverter 6 senses changes of magnetic poles and energizes coils 4a, 4cand coils 4b, 4d, alternatively, keeping the rotor rotating in onedirection.

FIGS. 12 A and B, respectively, illustrate the surface distribution ofmagnetic flux density for the rotor main magnet 7 and that for the rotorposition sensing magnet 8.

A brushless dc motor described above of the prior art construction ischaracterized by stable motor driving torque and small detent torque buthas the following disadvantages.

The special magnetic pattern of the rotor magnet makes it difficult tomass-produce motors of uniform characteristics. The prior art requiresnon-magnetized areas to be formed on the rotor magnet, and for thispurpose, requires a special yoke-shaped magnetizing head. Also thedifference in magnetizing devices (magnetization power supply,magnetization voltage, electrostatic capacity of power supply, etc.) orchanges of surrounding temperature cause a dispersion of spatialdistribution of magnetic flux density. Being largely dependent on themagnetized pattern of the rotor magnet, motor characteristics varyacutely when space distribution of magnetic flux density is uneven.

In addition, in a construction where a magnetoelectricity converter isused for rotor position sensing, a rotor position sensing magnet 8 needsto be used in addition to a torque-generating rotor main magnet 7. Thismakes the rotor construction even more complicated.

The present invention has for its object eliminating the problems of theprior art as thus far described, and relates to a brushless dc motor anda rotor magnet to be used therein, wherein the rotor magnet is entirelymagnetized with no non-magnetized areas required, thereby facilitatingthe magnetization, and whereby space distribution of magnetic fluxdensity is less uneven.

The present invention, which eliminates the aforementioned technicalproblems, relates in a preferred embodiment to (1) a brushless dc motorcomprising a stator having magnetic poles with coils and inter-poles, arotor made of a cylindrical magnet radially magnetized, and a rotorposition sensor, wherein the rotor rotates around the stator forming aradial flux structure, and the rotor magnet has magnetic grooves on itsinner surface facing the stator and is entirely magnetized, and (2) arotor magnet to be used therein.

"Magnetic grooves" mentioned herein refers not only to apparent groovesin the ordinary sense, but also to an instance where non-magnetizedreinforcements are filled in such grooves and even to an instance whereplastic magnets are filled in the grooves cut in a high-performancemagnet, such as a rare-earth magnet. "A rotor magnet" mentioned hereinincludes one made of multiple materials being formed into a nearlycylindrical shape, as well as one of completely uniform structure. Theterm "cylindrical" used herein has a broader sense and includesreference to being ring-shaped, etc.

In this invention, the entire rotor magnet is completely magnetizedwithout having any non-magnetized area, and the rotor has magneticgrooves in its surface opposite the stator. Because of this, if therotor magnet and the stator are assembled into a motor, the desiredspace magnetic flux density is obtained by changing its permeance in acircumferential direction. The stator with interpoles and the rotormagnet with a specific space magnetic flux density are combined toproduce torque at any position of the rotor and to ensure stablestarting.

In this invention, only one magnetoelectricity converter, e.g., Halleffect device, installed in the stator surface opposed to the rotor, isenough to detect the position of the rotor, and the invention does notrequire a rotor position detection magnet as required by theconventional technology.

In the accompanying drawings:

FIG. 1 is a schematic cross-sectional view of a brushless dc motoraccording to this invention;

FIG. 2 is a detail of the rotor magnet shown in FIG. 1,;

FIG. 3 is a graph illustrating the surface distribution of magnetic fluxdensity as to the magnet;

FIG. 4 is an axial cross-sectional view of the fan-motor to which thisinvention is applied, as an illustrative embodiment;

FIG. 5 A-E show illustrative sectional shapes of the grooves cut in therespective rotor magnet;

FIGS. 6 and 7 show different shapes, in sectional view, of the rotormagnet;

FIGS. 8 and 9 show different structures in axial sectional views of therotor;

FIG. 10 is a sectional view of a conventional motor;

FIG. 11 is a detail of the rotor magnet; and

FIGS. 12 A and B are graphs illustrating the difference in surfacedistribution of magnetic flux density between the rotor position sensingmagnet A and the rotor main magnet B.

FIG. 1 is a cross-sectional view of a brushless dc motor according tothis invention, and an example of a 4-pole outer-rotor-type motor. Asthe basic parts of the stator section are almost the same as those ofthe conventional technology shown in FIG. 10, its corresponding partsare shown with identical reference numerals for easy reference.

The stator includes one stator yoke 2, four stator magnetic poles 3a,3b, 3c, and 3d, which are positioned at a spaced angle of 90 degrees insuccession to the stator yoke, four coils 4a, 4b, 4c, and 4d, which arewound around the stator magnetic poles, and four interpoles 5ab, 5bc,5cd, and 5da, which are positioned between the stator magnetic poles.The magnetoelectricity converter 6, which is used to detect the positionof the rotor is, for example, a Hall element or the like, and it isinstalled to face a below-mentioned rotor magnet 11 in the vicinity ofthe top of interpole 5bc.

The rotor includes rotor yoke 10 and rotor magnet 11, and it is mountedso as to rotate freely about a central axis (not shown). The details ofrotor magnet 11 are shown in FIG. 2, and its surface magnetic fluxdensity in FIG. 3. Rotor magnet 11 is radially 4-pole-magnetized and hastwo grooves 12a and 12b. In the case of four poles, a circular arc of 2theta degrees is formed with a magnetic pole switching point (neutralpoint) as a symmetrical axis having theta degrees each at both sidesthereof. Due to the magnetic grooves 12a and 12b, the surface magneticflux density of rotor magnet 11 is distributed in the direction of thecircumference, as shown in FIG. 3. That is, at the grooves 12a and 12bas compared to other parts, the magnetic flux density on the surfacewith which the stator magnetic poles interlink becomes extremely low andcomes close to the surface magnetic density distribution shown in FIG.12B on the conventional rotor main magnet. On the other hand, as theneutral points come to the position of 90 degrees in the case of 4-pole,an additional rotor position detector magnet for the conventionaltechnology, as shown in FIG. 10, is not required. If the groove width ofrotor magnet 11 is 2 thetas as viewed from a central angle, then thearrangement of the grooves and the magnetic poles is as follows, and theunit of the angles is degrees.

N pole (90 - Theta); S pole (90 - Theta); Groove S Pole (Theta); GrooveN pole (Theta); N pole (90 - Theta); S pole (90 - Theta); Groove S pole(Theta); Groove N pole (Theta).

This invention is characterized by the following: the magnetic groovesare formed so as to give magnetic changes in a circumferentialdirection, and the rotor magnet is so constructed that its whole can bemagnetized without having any non-magnetized area, and this eliminatesthe need for both a rotor main magnet and a rotor position detectionmagnet.

The operating principle of such a brushless dc motor constructed asabove is the same as that of the aforementioned conventional technology.With the position relationship between rotor magnet 11 andmagnetoelectricity converter 6 fixed as shown in FIG. 1, if coils 4b and4d are energized, and stator magnetic poles 3b and 3d, and the otherstator magnetic poles and interpoles are excited to N and S poles,respectively, the N pole of rotor magnet 11 and the N poles of statormagnetic poles 3b and 3d repel each other magnetically and the rotorrotates counterclockwise (in the direction of the arrow in FIG. 1). Whenthe rotor rotates to 90 degrees, coils 4b and 4d are de-energized, andat the same time, coils 4a and 4c are energized. This causes statormagnetic poles 3a, 3c, and the other stator magnetic poles andinterpoles to be excited to N and S poles, respectively. As a result,the rotor continues to rotate counterclockwise because the N pole ofrotor magnet 11 and the N poles of stator magnetic poles 3a and 3c repeleach other magnetically.

At this point interpoles 5ab, 5bc, 5cd and 5da function as a generatorof torque (no dead point) at any position of the rotor.

As explained above, each time the rotor rotates through 90 degrees,magnetoelectricity converter 6 detects the changes of the magnetic polesin rotor magnet 6 and energizes a pair of coils 4a and 4c, and a pair ofcoils 4b and 4d, alternatively. This causes the rotor to rotatecontinuously in one direction.

In connection with this invention, an experiment was made for rotormagnet 11 shown in FIG. 2, and for comparison a conventional-type rotormagnet, to find the effects by groove angles of theta on thecharacteristics of the motor. In this experiment, with the statorconstruction fixed, and 40 degrees set as a groove 12a/12b angle oftheta, when a motor axis torque of 10g-cm was applied, the rate ofrotation and the motor current values were obtained. Table 1 shows theresults and the motor efficiency based on them.

                                      TABLE 1                                     __________________________________________________________________________            Conventional                                                                         Invented Technology                                            Characteristics                                                                       Technology                                                                           Theta = 15                                                                          Theta = 20                                                                          Theta = 30                                                                          Theta = 40                                   __________________________________________________________________________    Rate of 3250   3080  3200  3560  3300                                         Rotation                                                                      (rpm)                                                                         Motor    140    125   125   118   125                                         Current                                                                       (mA)                                                                          Motor    20       21.1                                                                                21.9                                                                              26    23                                          Efficiency                                                                    (%)                                                                           __________________________________________________________________________

Table 1 shows that compared with the conventional technology, thisinvention produces a low current value and a rise in efficiency of 1 to6%. If a 4-pole magnetized rotor magnet is used, it became clear thatthe motor is most efficient in the vicinity of theta=30 degrees. Theexperiment shown in Table 1 was made based on the relationship of t1=4t2in which the gap between the rotor magnet and the stator is t1, and thegap between the groove and the stator is t2. This is determined by themagnetic flux density distribution and the experiment results.

Therefore, it is desirable that the shape of the groove formed in therotor magnet should be designed to satisfy the following two formulas:

    0.3×360 / P<2 Theta <360 / P                         (1)

    D≧G                                                 (2)

Where:

2 Theta: Central angle of the circular arc with a magnetic poleswitching point (neutral point) as a symmetrical axis having Θ degreesat each side thereof

P: Number of magnetized poles

D: Center depth of the groove

G: Minimum gap between the stator and the rotor magnet

The brushless motor of this invention is applicable for many purposes.Good starting and high efficiency are the characteristics of the motor,which is suitable for products such as fan motors.

FIG. 4 shows the fan motor. FIG. 4 has reference numerals in common withFIG. 1 for easy explanation of the motor section. A rotor yoke 10 is setinside an impeller 40, molded in one piece with synthetic resin, afterwhich both members are heat-clinched and fastened. Rotor shaft 18 isfree to rotate in bearings 44 and 45 fitted into the housing 42, and isheld in position with a retaining ring 48 after a spring 46 is adjustedto a given preload. The stator, made from coils 4a-4d wound on thestator yoke 2, laminated and then insulated, is fixed to the outside ofa bearing holder located in the center of the housing 42.

Circuit board 50 is fastened with screws 52 to the inner side of housing42. A logic circuit for coil excitation, switching elements, andmagnetoelectricity converter 6 for rotor position sensor etc., aremounted on the circuit board 50, to which the terminal of each coil4a-4d is also connected. The required power is supplied from an outsidesource by means of wire 52.

The groove cut in the rotor magnet is not limited to a rectangular shapeas viewed in cross-section. There may be other different shapes, such asA˜E on FIG. 5.

Example A in FIG. 5 shows a V-shape grooved structure;

Example B in FIG. 5 is a semicircle;

Example C in FIG. 5 is a groove having an arc shape on each side of itsbottom;

Example D in FIG. 5 is a groove of which the bottom is curved out ofcentral angle Theta on both edges; to form a swelled curve toward theface of the stator.

Other different shapes of groove may be possible.

The rotor magnet structure can also possibly by changed. FIGS. 6 andFIG. 7 illustrate this. To improve the characteristics, the rotor magnetmay be made up of a high performance magnet, such as a sintered or arare-earth magnet. If such material would be used for the whole of therotor magnet, however, it would be costly, far from cost-effective inmany cases. Therefore, FIG. 6 shows that high quality magnets, such assintered or rare-earth magnets, should be used in the desired locationof, and as part of, the rotor magnet and that cheaper and easy-to-moldplastic magnets 51 be used in the desired shape for the remaining part.The structure shown in FIG. 6 enables production of a motor having highperformance with respect to cost.

In the region of the grooves, the rotor magnet is thinner incross-section and hence its strength is reduced in the vicinity of thegrooves and therefore subject to deformation. Attention should be paidto handling it. FIG. 7 is a structure designed for increasing the magnetstrength with proper reinforcement material 53 filling the grooved partof the rotor magnet 54. For instance, non-magnetic plastics serve thepurpose of reinforcement 53. Moreover, plastic magnets are adaptable asreinforcement 53 for the rotor magnet 54 in which high performancemagnets, such as sintered or rare-earth magnets are used.

FIG. 8 and FIG. 9 show a practical structure of the rotor. If theimpeller must be joined with the rotor, like a fan-motor for instance,it should be designed to make a combination of rotor magnet 56, shaft55, and reinforcement 57 (impeller for fan-motor) as an integralmolding, as in FIG. 8.

FIG. 9 is another combining form in which rotor magnet 56 and shaft 55are joined by reinforcement 58 that incorporates gear teeth on thecircumference.

Such examples of structure are not all that this invention covers. Thetype explained above is an outer-rotor type motor. In like manner, aninner-rotor type can be made up in combining form, with high efficiencygained as well, irrespective of a driving system or a number of magneticpoles. Of course, it is possible to make changes of the magnet, thereinforcement material, and the components structured by thereinforcement.

This invention is applicable to a photoelectro-type rotor positionsensor system in addition to a magnetoelectricity converter rotorposition sensor system.

This invention employs a rotor magnet which is entirely magnetized,without non-magnetized areas, does not require any magnetizing jig in aspecial yoke-shape, and accordingly facilitates in designing the jig,and nearly eliminates the dispersion resulting from magnetization powersupply, such as magnetization voltage, capacity of magnetization powersupply, etc., and provides for more steady space distribution ofmagnetic flux density. And, some proper change in thickness or space gapof the rotor magnet allows optional setting of space distribution ofmagnetic flux density.

This invention is designed for the rotor position sensor to work underthe influence of the magnetized pattern of the rotor magnet contributingto the torque. The rotor structure, therefore, becomes very simple, andrequires no additional rotor position sensing magnet as previously used.

In this invention, compared with previous types, operating currents areso small that high efficiency is gained and the temperature can be keptfrom going up. It contributes to material cost reduction because therotor magnet can be made smaller in capacity while having the samefunctional performance capability as the previous type. Moreover, ithelps to cut back the cogging torque. That is how the brushless dc motorrelated to this invention produces very significant effects on thefacilitation in manufacture, the characteristic improvement infunctional performance, and only a small degree of dispersion inproperties.

The invention has been shown and described in preferred form only, andby way of example, and many variations may be made in the inventionwhich will still be comprised within its spirit. It is understood,therefore, that the invention is not limited to any specific form orembodiment except insofar as such limitations are included in theappended claims.

We claim:
 1. A brushless dc motor comprising:a stator having magneticpoles carrying coils, and interpoles between the poles, a cylindricalmagnet rotor rotatable with respect to the stator, the rotor beingradially magnetized to define at least four poles around its periphery,the rotor being completely magnetized so as to leave no non-magnetizedareas between the poles of the rotor, and the rotor having substantiallyaxial grooves in its surface facing the stator, each groove beinglocated closer to one end of one of the rotor poles than to the otherend, said other end of that rotor pole being ungrooved, and each of thegrooves extending circumferentially into two adjacent poles of themagnet.
 2. A brushless dc motor as defined in claim 1 including a rotorposition sensor, the sensor being a magnetoelectricity converterpositioned adjacent to the surface of the rotor facing the stator.
 3. Abrushless dc motor as defined in claim 1, wherein the shape of thegrooves in the rotor magnet is represented by the following twoformulas:

    0.3×360 / P<2 Θ<360 / P

    D≧G

where; 2Θ: width (degrees) of the groove P: number of magnetized poles D: depth at the center of the groove G : the smallest gap between thestator and the rotor magnet.
 4. A brushless dc motor as defined in claim1 wherein the grooves extend in the axial direction of the rotor magnet,and the grooves extend circumferentially into two adjacent poles of therotor.
 5. A brushless dc motor as defined in claim 1 wherein the rotormagnet is formed of two different magnetic materials.
 6. A brushless dcmotor as defined in claim 1 wherein nonmagnetic reinforcement materialfills the grooves.
 7. A brushless dc motor as defined in claim 1 whereinthe rotor comprises a rotor body and a rotor magnet, the magnet beingembedded within the rotor body.
 8. A hollow cylindrical rotor magnet,for a brushless dc motor, formed on a single piece of permanentlymagnetized material, the magnet having sectors magnetized radially inopposite directions to define at least four poles of opposite polarityextending around the circumference of the rotor, and the magnet havingsubstantially axial grooves in its inner cylindrical surface extendingalong the length of the magnet, each groove being located closer to oneend of one of the rotor poles than to the other end, said other end ofthat rotor pole being ungrooved, and each groove extendingcircumferentially into two adjacent poles of the magnet.
 9. A magnet asdefined in claim 8 wherein each groove extends circumferentially intotwo adjacent poles of the magnet.
 10. A magnet as defined in claim 8wherein the magnet is completely magnetized so as to leave nonon-magnetized sectors between the poles of the magnet.